In recent years, flat panel displays are used in various commodity products and fields, and thus they are required to have a large size, high image quality and low power consumption.
Under the circumstances, organic EL display devices, which include an organic EL (electroluminescent) element that utilizes electro luminescence of an organic material, are attracting great attention as all-solid state flat panel displays that are excellent as having capability of low voltage operation, quick responsivity and light emission.
In an active matrix type organic EL display device, for example, a thin-film organic EL element is provided on a substrate having TFTs (thin-film transistors). The organic EL element includes an organic EL layer (including a light emitting layer) that is disposed between a pair of electrodes. The TFTs are connected to one of the pair of electrodes. A voltage is applied across the pair of electrodes so that the light emitting layer emits light, thereby displaying images.
In the above conventional organic EL display device, it is known that a sealing layer is formed on the organic EL element to prevent the degradation of the organic EL element due to moisture or oxygen.
Moreover, in the above conventional organic EL display device, it is known that a flexible base is used as a support substrate for supports an organic EL element to make the device repeatedly bendable.
In the above bendable organic EL display device, in order to protect an organic EL element and a sealing layer having low bendability, it is required to minimize a bending stress to be applied to the organic EL element and the sealing layer. Therefore, in the conventional bendable organic EL display device, generally the base is thick to adjust a neutral surface of the organic EL display device, thereby minimizing the bending stress.
However, in the above conventional organic EL display device, since the base is thick, heat from the organic EL element accumulates inside the device (e.g., base). This sometimes results in the dimensional difference between layers in the organic EL element and/or layers in the sealing layer inside the device due to difference in the thermal expansion coefficient. As a result, in the conventional organic EL display device, separation of films in the layers in the organic EL element and/or the layers in the sealing layer, or twist of the base occurs, and this damages the organic EL display device.
To cope with this, in the above conventional organic EL display device, as described in Patent Document 1 below for example, a plastic substrate is used as a base, a plurality of through holes are formed in the plastic substrate, and heat dissipating layers are filled into the through holes, thereby preventing local accumulation of heat from the organic EL element and controlling local temperature rise.
Also, in the conventional organic EL display device, as described in Patent Document 2 below for example, a metal substrate is used as the base, thereby improving heat dissipating properties and dissipating heat from the organic EL element efficiently.